Prostate cancer is the most common nonskin cancer and second most common cause of cancer mortality in US men. Most prostate cancer is initially androgen dependent (AD). Prostate cancer cells initially require androgen for continued proliferation. Response to ablation of testosterone through androgen deprivation therapy (ADT), either surgically (orchiectomy) or medically (GnRH agonists or estrogens), leads to rapid induction of apoptosis of sensitive prostate cancer cells. The positive response rate is about 86% based on decrease in prostate specific antigen (PSA) and stabilization or decrease in tumor volume. The cell death that occurs generally takes place within the first few days to a week. However, the positive response is followed by a period of growth arrest in which remaining cells tend not to die. After 18-36 months following hormone ablation, growth recurs in 90% of cases. Invariably, surviving cancer cells become androgen independent or unresponsive, and androgen-independent (AI) tumor growth follows. Since ADT is initially very effective, a therapy that could take advantage of the benefits of ADT and extend or enhance its effects would be of great benefit.
Androgen independence appears to arise by a variety of mechanisms. Mutations in the androgen receptor gene are rare at diagnosis, but increase after exposure to the anti-androgen flutamide. However, these mutations do not occur in the majority of patients and do not explain most cases of hormone-refractory disease. High levels of bcl-2 are seen with greater frequency in advanced disease as compared to localized disease. Thus, the ability to induce apoptosis diminishes as the disease progresses. The proliferation of cells harboring mutations of the tumor suppressor gene p53, the loss of TGF-β receptors, and the expression of peptide growth factors likely play a role in the development of a hormone-refractory state. However, these processes do not explain the rapidity and frequency of development.
The insulin-like growth factor receptor (IGF-IR) is a ubiquitous transmembrane tyrosine kinase receptor that is essential for normal fetal and post-natal growth and development. IGF-IR can stimulate cell proliferation, cell differentiation, changes in cell size, and protect cells from apoptosis. It has also been considered to be quasi-obligatory for cell transformation (reviewed in Adams et al., Cell. Mol. Life. Sci. 57:1050-93 (2000); Baserga, Oncogene 19:5574-81 (2000)). IGF-IR is located on the cell surface of most cell types and serves as the signaling molecule for growth factors IGF-I and IGF-II (collectively termed henceforth IGFs). IGF-IR also binds insulin, albeit at three orders of magnitude lower affinity than it binds to IGFs. IGF-IR is a pre-formed hetero-tetramer containing two alpha and two beta chains covalently linked by disulfide bonds. The receptor subunits are synthesized as part of a single polypeptide chain of 180 kd, which is then proteolytically processed into alpha (130 kd) and beta (95 kd) subunits. The entire alpha chain is extracellular and contains the site for ligand binding. The beta chain possesses the transmembrane domain, the tyrosine kinase domain, and a C-terminal extension that is necessary for cell differentiation and transformation, but is dispensable for mitogen signaling and protection from apoptosis.
IGF-IR is highly similar to the insulin receptor (IR), particularly within the beta chain sequence (70% homology). Because of this homology, recent studies have demonstrated that these receptors can form hybrids containing one IR dimer and one IGF-IR dimer (Pandini et al., Clin. Canc. Res. 5:1935-19 (1999)). The formation of hybrids occurs in both normal and transformed cells and the hybrid content is dependent upon the concentration of the two homodimer receptors (IR and IGF-IR) within the cell. In one study of 39 breast cancer specimens, although both IR and IGF-IR were over-expressed in all tumor samples, hybrid receptor content consistently exceeded the levels of both homo-receptors by approximately 3-fold (Pandini et al., Clin. Canc. Res. 5:1935-44 (1999)). Although hybrid receptors are composed of IR and IGF-IR pairs, the hybrids bind selectively to IGFs, with affinity similar to that of IGF-IR, and only weakly bind insulin (Siddle and Soos, The IGF System. Humana Press. pp. 199-225. 1999). These hybrids therefore can bind IGFs and transduce signals in both normal and transformed cells.
Endocrine expression of IGF-I is regulated primarily by growth hormone and produced in the liver, but recent evidence suggests that many other tissue types are also capable of expressing IGF-I. This ligand is therefore subjected to endocrine and paracrine regulation, as well as autocrine in the case of many types of tumor cells (Yu, H. and Rohan, J., J. Natl. Cancer Inst. 92:1472-89 (2000)).
The androgen receptor (AR) consists of 3 functional and structural domains: an N-terminal (modulatory) domain; a DNA binding domain (Interpro Accession No. IPR001628) that mediates specific binding to target DNA sequences (ligand-responsive elements); and a hormone binding domain. The N-terminal domain (NTD) is unique to the androgen receptors and spans approximately the first 530 residues; the highly-conserved DNA-binding domain is smaller (around 65 residues) and occupies the central portion of the protein; and the hormone ligand binding domain (LBD) lies at the receptor C-terminus. In the absence of ligand, steroid hormone receptors are thought to be weakly associated with nuclear components; hormone binding greatly increases receptor affinity. The interaction among androgen receptor (AR), androgen, and prostate cancer is complex. Distribution of AR between the nucleus and cytoplasm is affected by androgen and androgen withdrawal. For example, AR immunoreactivity is observed only in the nuclei of LuCaP 35 cells grown in intact male mice, but strong immunoreactivity is observed in the cytoplasm and nuclei of LuCaP 35 grown in intact male mice and subsequently castrated.